The drilling of holes in rock for placement of support bolts or for other purposes has in the past generally involved drilling holes of only a single diameter. Recently in connection with the art of fastening support bolts to the roofs of mines it has become necessary to drill different diameter holes in close proximity. For example, conditions could warrent the use of one-inch diameter holes for three-quarter inch rebar bolts epoxy-anchored and 13/8" diameter holes for expansion-anchored bolts. Present drilling technology requires changing both the drill and the drive when hole size is changed, a time-consuming and expensive procedure. There has, therefore, been a need for a drilling system which would allow changing hole size simply by changing the drill and bit without also requiring change of the entire drill drive mechanism.